There are many different methods used to produce fluid from an oil well. Some wells require no pumping at all. These types of wells are called “free flowing” and are usually highly desirable by oil production companies. Most wells, however, are not free-flowing wells. Most wells require some sort of method to lift oil or other fluid from the well and to the surface. These methods are broadly included in a wide spectrum of methods called “artificial lift.” Artificial lift is needed in cases when wells are not free-flowing at all, or are free-flowing but determined to be insufficiently free-flowing. There are many different types of artificial lift pumping systems. The type of artificial lift that is relevant to our device is pumping units used in reciprocating rod-lift pumping systems. A pumping unit providing this artificial lift is driven by an alternating current (AC) electric motor energized by alternating current from an AC electric power grid. Some pumping units are located where there is no electricity available. In those cases, the pumping unit may be driven by an IC (Internal Combustion) engine. There are many pumping units powered with IC engines. Our device does not apply to such IC engine drive pumping units.
A well manager unit is ordinarily used to monitor and regulate the operation of the oil well in response to conditions in the well. For example, well parameters such as the speed of the motor, the amount of fill of the pump, amount of gas in the well, down-hole well pressure, etc. are monitored and controlled as required. The commonly used rod pumps are a long-stroke pumping unit and a beam pumping unit. Many, in fact the majority, of pumping units do not require speed regulation. These pumping units operate at an average speed that is fixed, typically driven by an AC Motor. These pumping units are controlled by a well manager by ON/OFF control. When the AC Motor is “on,” it runs at a fixed average speed. When the AC Motor is “off,” the speed is fixed at zero. The well manager will “regulate” the well by controlling the amount of “off” time versus “on” time. This is often called “duty-cycle” control.
Both the average speed of a pumping unit and its instantaneous speed must be taken into consideration when operating the pumping unit in the best way under the prevailing well conditions. The primary reason for modulating the average speed of a pumping unit is to control the volume of fluid produced by the pumping unit over a given period time. In other words, the pump takes out of the well all of the fluid that the well is capable of producing. In some cases, the pump may be oversized relative to the well. In those cases, the pumping unit may be required to slow down. Consequently, the well manager may slow down the average speed of the pumping unit. The primary reason for modulating the instantaneous speed of a pumping unit is to avoid creating rod compression, excessively high rod tension, excessive rod tension gradients, excessively low rod tension, mechanical stress in the pumping unit or otherwise damaging equipment. In some cases, it is necessary to regulate the speed of the electric motor to avoid creating compression of the pumping unit's rod or otherwise damaging equipment. This may require braking to slow the motor speed and then increasing the motor speed, depending on the position of the rod during the course of each stroke cycle. Each stroke cycle includes an upstroke to a predetermined top rod position where the direction of movement of the rod reverses and begins a downstroke until the rod reaches a predetermined bottom rod position. Then the rod's upstroke is again initiated.
Normally braking is accomplished by directing electrical energy through resistors that dissipate this electrical energy as heat to the surrounding environment. This, however, is a fire hazard. It is also a waste of electrical energy. Some pumping units with AC motors and variable frequency AC drives operate without any braking at all. In these cases, the pumping units are operated at very low average and/or low instantaneous speeds. Or, if the pumping units are operated at higher speeds, mechanical damage is simply tolerated as a consequence of the additional stress.
Certain types of pumping units are more prone to damage at high speed operation without braking. Other types of pumping units are less prone to damage at high speed operation without braking. The type of braking produced by an AC motor with a variable frequency AC drive is sometimes called “dynamic braking.” This is done to distinguish the two main types of brakes, “dynamic brakes” and “holding brakes.” All pumping units are equipped with mechanical holding brakes that hold the pumping unit in position when the holding brake is engaged. Dynamic braking is the process of the AC motor, under the control of the variable frequency AC drive, removing energy from the mechanical system thereby slowing or retarding the motor shaft's rotation. The variable frequency AC drive converts this energy into heat, when the braking method is resistive. In addition to all of the reasons listed: In standard practice, when braking resistors are used, the braking resistors are usually not adequately sized to dissipate the necessary amount of energy to allow for optimum pumping unit control. Use of braking resistors involves a compromise between the size and cost of braking resistors and associated electrical components and pumping unit performance.
This background discussion is not intended to be an admission of prior art.